The use of Doppler echography presents a difficulty when this technique is used for measuring the local velocities of a fluid at a high temperature or moving inside a conduit having a thick wall. In such cases, the transducer (or transducers) used for emitting the ultrasounds and for receiving their echo is (are) located at a significant distance from the liquid, either because of the thickness of the wall, or because of the need to insulate the transducer(s) from heat. As a result, the usual division into observation channels concerns a length which can also be important in comparison with the width of the zone of interest to be examined, and because the number of channels available is limited, the resolution is diminished.